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Description: Experimental realizations of model magnetic systems--ones in which the interactions between the magnetic ions have a particularly simple form--are test-beds for our fundamental understanding of magnetic phenomena. If we are to understand the unusual properties of complex and potentially useful materials in which magnetism plays an important role, such as colossal magnetoresistive oxides or high temperature superconductors, we must also understand the properties of simple systems. SrCuO{sub 2} is an example of a one-dimensional magnetic system in which the characteristic energy of excitations from the magnetic ground state is comparable to that of charge motion along the chains. Our results show that the spectrum of magnetic excitations is nevertheless well described by that for the limiting case when charge and spin dynamics are separable.
Date: November 19, 2003
Partner: UNT Libraries Government Documents Department

Structural study of Sr{sub 2}CuO{sub 3+delta} by neutron powder diffraction

Description: Average crystal structures of superconducting Sr{sub 2}CuO{sub 3+{delta}} synthesized at ambient pressure from a hydroxometallate precursor were refined from neutron powder diffraction data. A simplified model was used to fit the modulated superstructures. Both compounds have an oxygen deficient La{sub 2}CuO{sub 4}-type tetragonal T structure with O vacancies located in the CuO{sub 2} planes, not in the Sr{sub 2}O{sub 2} layers. This raises important questions about the superconductivity in Sr{sub 2}CuO{sub 3+{delta}} reported to be a 70 K superconductor.
Date: November 1, 1996
Creator: Shimakawa, Y.; Jorgensen, J.D.; Mitchell, J.F.; Hunter, B.A.; Shaked, S.; Hinks, D.G. et al.
Partner: UNT Libraries Government Documents Department

Surface state and normal layer effects in high T{sub c} superconductors

Description: In addition to the conducting CuO{sub 2} (S) layers, most high-{Tc}, superconductors also contain other conducting (N) layers, which are only superconducting due to the proximity effect. The combination of S and N layers can give rise to complicated electronic densities of states, leading to quasilinear penetration depth and NMR relaxation rate behavior at low temperatures. Surface states can also complicate the analysis of tunneling and photoemission measurements. Moreover, geometrical considerations and inhomogeneously trapped flux are possible explanations of the paramagnetic Meissner effect and of corner and ring SQUID experiments. Hence, all of the above experiments could be consistent with isotropic s-wave superconductive within the S layers.
Date: January 1, 1996
Creator: Klemm, R.A.; Ledvij, M. & Liu, Samuel H.
Partner: UNT Libraries Government Documents Department

Enhanced thermal conductivity through the development of nanofluids

Description: Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids required in many industrial applications. To overcome this limitation, a new class of heat transfer fluids is being developed by suspending nanocrystalline particles in liquids such as water or oil. The resulting nanofluids possess extremely high thermal conductivities compared to the liquids without dispersed nanocrystalline particles. For example, 5 volume % of nanocrystalline copper oxide particles suspended in water results in an improvement in thermal conductivity of almost 60% compared to water without nanoparticles. Excellent suspension properties are also observed, with no significant settling of nanocrystalline oxide particles occurring in stationary fluids over time periods longer than several days. Direct evaporation of Cu nanoparticles into pump oil results in similar improvements in thermal conductivity compared to oxide-in-water systems, but importantly, requires far smaller concentrations of dispersed nanocrystalline powder.
Date: November 1, 1996
Creator: Eastman, J.A.; Choi, U.S.; Li, S.; Thompson, L.J. & Lee, S.
Partner: UNT Libraries Government Documents Department

Joining of melt-textured YBCO : a direct contact method.

Description: We report a method for making weld joints, capable of transmitting high supercurrent densities, in bulk samples of melt textured YBCO. The joining procedure is carried out in a flowing atmosphere of O{sub 2} to eliminate problems associated with nitrogen gas, which can become trapped in the joint. No filler or fluxing material is used. The method can be used to join large areas (several cm{sup 2}) that are capable of transmitting supercurrent densities exceeding 10{sup 4} A/cm{sup 2}.
Date: December 19, 2001
Creator: Chen, L.; Claus, H.; Paulikas, A. P.; Zheng, H. & Veal, B. W.
Partner: UNT Libraries Government Documents Department

Semiconductor bridge, SCB, ignition studies of Al/CuO thermite

Description: The authors briefly summarize semiconductor bridge operation and review their ignition studies of Al/CuO thermite as a function of the capacitor discharge unit (CDU) firing set capacitance, charge holder material and morphology of the CuO. Ignition thresholds were obtained using a brass charge holder and a non-conducting fiber-glass-epoxy composite material, G10. At - 18 C and a charge voltage of 50V, the capacitance thresholds were 30.1 {mu}F and 2.0 {mu}F respectively. They also present new data on electrostatic discharge (ESD) and radio frequency (RF) vulnerability tests.
Date: April 1, 1997
Creator: Bickes, R.W. Jr.; Wackerbarth, D.E. & Mohler, J.H.
Partner: UNT Libraries Government Documents Department

Meissner holes in remagnetized superconductors

Description: Flux distributions on different faces of YBCO single crystals remagnetized by unidirectional and rotating fields are studied using advanced magneto-optical techniques. Unusual structures corresponding to the appearance of strong current concentration along certain fronts are found. At such fronts the vortex lines bend into closed loops which then collapse and form flux free cylinders. Strong magnetization currents along these Meissner holes result in essential flux redistribution and development of dynamical instabilities in the samples.
Date: April 1996
Creator: Vlasko-Vlasov, V. K.; Welp, U. & Crabtree, G. W.
Partner: UNT Libraries Government Documents Department

Parallel solution of the three-dimensional time-dependent Ginzburg- Landau equation

Description: The recent discovery of superconductivity in a class of copper-oxide compounds (the cuprate superconductors) at liquid nitrogen temperatures has generated a renewed interest in the magnetic properties of type-II superconductors. In our work, we are investigating these properties using the phenomenological time- dependent Ginzburg-Landau equation. This paper describes the parallelization of this equation.
Date: August 1, 1996
Creator: Galbreath, N.; Gropp, W.; Gunter, D.; Leaf, G. & Levine, D.
Partner: UNT Libraries Government Documents Department

The nature of twin boundaries in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: Twin boundaries are the most commonly observed lattice defect in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. Furthermore, the region around a twin boundary for which the structure and composition are seriously affected is of a scale comparable to the coherence length for the superconducting order parameter. Thus, twin boundaries can be important in the behavior of magnetic vortices and the critical current density in this material. In this paper the authors review the results of a wide range of investigations of twin boundary structure and composition by advanced transmission electron microscopy methods, both imaging and analytical in nature. A simple Landau model of twin boundary energy and width is proposed.
Date: June 1, 1999
Creator: Zhu, Y. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

A valence-fluctuation mechanism with highly anisotropic S-wave pairing

Description: The author has developed a fairly ab initio theory of cuprate superconductivity, based on the concept that the above-T{sub c} state is a normal Fermi liquid with strong electronic correlations of the type found in valence-fluctuation (VF) and heavy-fermion materials. The ground-state-correlation aspect is found to provide a satisfactory source or mechanism for the pairing. In addition to the high T{sub c}`s this explains many of the unusual features of cuprate superconductivity.
Date: April 1, 1995
Creator: Brandow, B.H.
Partner: UNT Libraries Government Documents Department

Thermodynamic and nonstoichiometric behavior of the lead-doped and lead-free Bi-2212 systems

Description: EMF measurements of oxygen fugacities as a function of stoichiometry were made on pb-doped and Pb-free superconducting ceramics at 700-815 C using oxygen titration. Equations of oxygen partial pressure vs composition and temperature were derived from the EMF measurements. Thermodynamic assessments of the partial molar quantities {Delta}{bar H}(O{sub 2}) and {Delta}{bar S}(O{sub 2}) for Pb-doped Bi- 2212 and Pb-free Bi-2212 indicate that the solid-state decomposition of these Bi cuprates at low oxygen partial pressure can be represented by the diphasic CuO-Cu{sub 2}O system. Comparison of these results with Pb-doped Bi-2223 in powder and silver sheath form is presented.
Date: February 1, 1997
Creator: Tetenbaum, M.; Hash, M.; Tani, B.S.; Luo, J.S. & Maroni, V.A.
Partner: UNT Libraries Government Documents Department

Neutron scattering study in the spin-1/2 ladder system: Sr{sub 14}Cu{sub 24}O{sub 41}

Description: Inelastic neutron scattering measurements were performed on the S=1/2 quasi-one-dimensional system Sr{sub 14}Cu{sub 24}O{sub 41}, which has both simple chains and two-leg ladders of copper ions. We have observed that both the chain and the ladder exhibit a spin gap, which originates from a dimerized state.
Date: October 1, 1996
Creator: Matsuda, M.; Katsumata, K.; Shapiro, S.M. & Shirane, G.
Partner: UNT Libraries Government Documents Department

HTS coil development and fabrication. Final report

Description: The objective of this subtask (Task 2C) was to develop high-temperature superconductor (HTS) coil technology aimed specifically at superconducting generator applications. Bi-2223 tape produced in a separate subtask (Task 2A) was first wound and tested in a small circular coil. This small coil winding experience led the authors to develop a tape strengthening method using a lamination process and to develop a turn insulation method using a paper wrap process. A prototype racetrack coil was wound using 2500 feet of the laminated and insulated Bi-2223 tapes. The racetrack coil was cooled to 20K in a vacuum dewar using a unique closed-cycle helium gas refrigeration system.
Date: October 1, 1996
Creator: Herd, K.G.; Salasoo, L. & Ranze, R.
Partner: UNT Libraries Government Documents Department

Practical superconductor development for electrical power applications: Quarterly report for the period ending December 31, 1999

Description: This is a multiyear experimental research program focused on improving relevant material properties of high-T{sub c} superconductors (HTSS) and on development of fabrication methods that can be transferred to industry for production of commercial conductors. The development of teaming relationships through agreements with industrial partners is a key element of the Argonne (ANL) program. Recent results on substrate deposition for coated conductors, vortex studies, development of hardened Ag-alloy sheaths for powder-in-tube conductors, and sol-gel processing of NdBa{sub 2}Cu{sub 3}O{sub x} (Nd-123) are presented.
Date: February 2, 2000
Partner: UNT Libraries Government Documents Department

Effect of defects on the critical points in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: The upper and lower critical points are investigated in untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals with dilute columnar defects. Dilute columnar defects raise the upper critical point, indicating that the transition near the upper critical point is a vortex entanglement transition. The lower critical point is very sensitive to columnar defect disorder and its position can be described by a Lindemann-like criterion similar to that for melting. Dilute columnar defects induce non-linear behavior in the I-V curves of the vortex liquid state above the lower critical point, which the authors interpret as a vestige of the critical region associated with the Bose glass transition below the lower critical point.
Date: January 26, 2000
Creator: Kwok, W. K.; Olsson, R. J.; Karapetrov, G.; Paulis, L. M.; Moulton, W. G.; Hofman, D. J. et al.
Partner: UNT Libraries Government Documents Department


Description: As a result technical and cost evaluations in this period, a decision has been made to redirect a portion of the experimental program. The 8/94 Phase II Test Plan included the following experimental tasks: (1) Reburn development at a scale of 15 million Btu/hr, at the University of Utah; (2) large burner testing at 100 million Btu/hr, slag tap firing with reburning, at the DB Riley Research Center; (3) Bench scale copper oxide tests at Tecogen; (4) Copper oxide moving bed reactor testing at 5 million Btu/hr, at the Illinois Coal Development Park (ICDP); and (5) Copper oxide reactor testing on a 30 million Btu/hr (approximately 3 MW{sub e} equivalent) slip stream at the DB Riley Research Center. A design for the 30 million Btu/hr copper oxide slipstream was completed in sufficient detail for cost and schedule quotations on major components. Both cost and construction time estimates were significantly higher than planned, a major factor being foundation and structural requirements specific to the available site. A further technical consideration was the limited, continuous test time available, due to operating restrictions, relative to the solids residence time within the reactor.
Date: January 1, 1996
Partner: UNT Libraries Government Documents Department